In various fields related to aircraft members, automotive members, wind power generating windmill members, sports goods and the like, structural materials that have been shaped by stamping molding sheet-like fiber-reinforced plastics are widely used. Such a fiber-reinforced plastic is formed by, for example, laminating plural sheets of a prepreg substrate that is obtained by impregnating reinforcing fibers with a thermoplastic resin, and integrating the laminate.
An example of the prepreg substrate is a product obtained by unidirectionally arranging continuous reinforcing fibers with a long fiber length in parallel, impregnating the arranged fibers with a thermoplastic resin, and forming the resultant into a sheet form. When a fiber-reinforced plastic formed from a prepreg substrate using such continuous long reinforcing fibers is used, a structural material having excellent mechanical properties can be produced. However, in this fiber-reinforced plastic, since continuous reinforcing fibers are used, fluidity at the time of shaping is low, and it is difficult to shape the fiber-reinforcing plastic into a complicated shape such as a three-dimensional shape. Therefore, in a case in which the fiber-reinforced plastic is used, the shape of the structural material thus produced is limited mainly to those shapes close to a planar shape.
Regarding the method for increasing fluidity at the time of shaping, for example, a method of cutting out plural prepreg pieces from a tape-like prepreg substrate having a narrow width into a constant length, dispersing the prepreg pieces in a planar form, integrating the prepreg pieces by press molding, and thus obtaining a sheet-like fiber-reinforced plastic, has been disclosed (Patent Document 1).
However, in this method, since the prepreg pieces are dispersed by causing prepreg pieces to fly by means of air or by spreading prepreg pieces in a liquid fluid and settling the prepreg pieces, it is very difficult to disperse the prepreg pieces uniformly such that the fiber axis directions of the reinforcing fibers are in completely random directions. Therefore, a fiber-reinforced plastic is obtained, in which mechanical properties such as strength vary depending on the position or direction even within the same sheet. In regard to structural materials, there is a high demand for materials in which there is less variation in the mechanical properties such as strength, and the mechanical properties are isotropic, or anisotropies thereof are under control. However, in this method, it is difficult to obtain a fiber-reinforced plastic in which mechanical properties are satisfactorily isotropic, or anisotropies thereof are under control, and there is less variation in the mechanical properties.
In addition, satisfactory heat resistance is also required from fiber-reinforced plastics. Generally, the heat resistance of a fiber-reinforced plastic is greatly affected by the heat resistance of the matrix resin used in the fiber-reinforced plastic. Typically, mechanical properties of a simple resin substance tend to deteriorate at a temperature higher than or equal to the glass transition temperature of the resin. Similarly, in the case of a fiber-reinforced plastic, mechanical properties tend to deteriorate at a temperature higher than or equal to the glass transition temperature of the matrix resin. In order to suppress this deterioration of mechanical properties to a minimum level, it is necessary to uniformly disperse reinforcing fibers in the matrix resin in the fiber-reinforced plastic. However, according to the method described above, in the process for integrating deposited prepreg pieces by heating, only molten matrix resin flows into the gaps between the deposited prepreg pieces. Therefore, in the fiber-reinforced plastic thus obtained, locally resin-rich portions are generated. Due to the effect of these resin-rich portions, a fiber-reinforced plastic obtainable by this method has a problem of inferior heat resistance.
Methods in which plural sheets of a prepreg substrate obtained by impregnating reinforcing fibers that are unidirectionally arranged in parallel with a thermoplastic resin and forming slits therein such that the slits intersect the fiber axes, are laminated and integrated to obtain a fiber-reinforced plastic, have also been disclosed (Patent Documents 2 to 6). In a fiber-reinforced plastic obtainable by this method, since slits are formed in the prepreg substrate and split the reinforcing fibers, satisfactory fluidity may be obtained at the time of shaping. Furthermore, when plural sheets of a prepreg substrate are laminated such that the fiber axial directions of the reinforcing fibers are not biased in a particular direction, for example, such that the fiber axial directions are shifted by 45° each when viewed in a planar view, a fiber-reinforced plastic having mechanical properties that are satisfactorily isotropic and have less variation can be obtained. Furthermore, anisotropy can be controlled by aligning the fiber axial directions in an arbitrary direction and laminating the plural sheets of prepreg substrate.
However, a fiber-reinforced plastic obtainable by this method has a problem that in a case in which stress occurs in a direction that follows the slit shape, these slit parts serve as the starting points of breakage, and mechanical properties deteriorate. Furthermore, since substantially only the resin exists in these slit parts, at a temperature higher than or equal to the glass transition temperature of the matrix resin, the fiber-reinforced plastic has a problem of inferior heat resistance, similarly to the method disclosed in Patent Document 1.
Furthermore, in this method, in a case in which a band-shaped fiber-reinforced plastic having satisfactorily isotropic mechanical properties is continuously produced, it is necessary to separately produce band-shaped prepreg substrates having fiber axial directions of the reinforcing fibers that are different from each other when viewed in a planar view (for example, 0°, 45°, 90°, and −45° with respect to the length direction), and to laminate those prepreg substrates. Therefore, the production process becomes complicated, with difficulty in control, and the production cost increases. Furthermore, even in a case in which sheets of a fiber-reinforced plastic are produced, the sheets need to be laminated while the respective prepreg substrate sheets are frequently rotated at predetermined angles of rotation (0°, 45°, 90°, and −45°) so that the fiber axial directions of the reinforcing fibers are not biased when viewed in a planar view. Therefore, similarly in this case, the lamination operation becomes complicated, with difficulty in control, and the production cost increases.
Patent Document 7 discloses a method for producing a fiber-reinforced plastic by dispersing reinforcing fibers by a papermaking process. In a fiber-reinforced plastic obtainable by this method, since the reinforcing fibers are almost uniformly dispersed, the fiber-reinforced plastic has excellent isotropic mechanical properties with less variation, and also has satisfactory heat resistance.
However, in a fiber-reinforced plastic obtainable by this method, since the reinforcing fibers are three-dimensionally entangled, fluidity at the time of shaping is very poor. Furthermore, the production process is also very complicated, and is markedly disadvantageous in terms of cost. In addition, in a case in which it is attempted to produce a fiber-reinforced plastic having a high percentage content of reinforcing fibers by this method, it is necessary to perform papermaking in a state in which the reinforcing fibers are more densely incorporated. However, when it is intended to impregnate reinforcing fibers that are subjected to papermaking at a high density as such with a matrix resin, since reinforcing fibers that are oriented in the thickness direction (direction of impregnation) in particular, among the reinforcing fibers that are three-dimensionally entangled, cope with the stress of the pressing force at the time of impregnation, pressure is not transferred to the resin, and it is very difficult to achieve impregnation. Furthermore, even in a case in which the fiber length of the reinforcing fibers is long, since three-dimensional entanglement becomes strong, similarly impregnation becomes difficult.